https://www.theverge.com/2018/7/10/17550316/apple-iphone-usb-restricted-mode-how-to-use-security

Apple quietly introduced a significant privacy safeguard as part of the new iOS 11.4.1 update that was released on July 9th. USB Restricted Mode prevents USB accessories that plug into the Lightning port from making data connections with an iPhone, iPad, or iPod Touch if your iOS device has been locked for over an hour. This seemingly small change goes a long way in blocking tools used by law enforcement to crack passcodes and circumvent Apple’s encryption and built-in measures designed to shield sensitive user data.

Apple describes it as a new “security protection” that’s introduced as part of iOS 11.4.1. The company hasn’t said as much, but it’s believed that USB Restricted Mode is Apple’s effort to combat devices like GrayKey that are specially designed to help law enforcement crack an iOS device’s passcode and retrieve data that would normally be guarded by encryption. iOS has built-in security features that prevent constant passcode guesses — such as when it locks down your device after several incorrect attempts. But companies have seemingly found a way of avoiding those safeguards via USB and the Lightning port. Now, Apple is trying to eliminate this method of gaining entry to a recovered or confiscated device.